This invention relates to measurement of blood hematocrit (Hct).
Hematocrit is the volume percent of red blood cells in a blood sample and is one of the most commonly performed blood tests. The standard method for measuring hematocrit involves collecting a blood sample in a capillary tube and centrifuging the tube to separate out the red blood cells from the plasma. By measuring the height of a resulting layer of red blood cells in the capillary and referencing it to the total blood volume, the volume percent of red blood cells can be quantified. Hematocrit measurements on blood samples are now often made with more automated techniques such as conductivity measurements.
Hematocrit can also be measured non-invasively. Two approaches have been reported for such non-invasive monitoring of hematocrit, an optical approach and an impedance method. Impedance methods are found to be inaccurate when protein and electrolyte levels are abnormal, such as when blood is replaced with crystalloid solutions, as would happen during resuscitation of trauma victims. All of the reported optical techniques are variations on oximetric methods where hematocrit is measured using only the concentrations of oxygenated and deoxygenated hemoglobin. The concentrations of oxygenated and deoxygenated hemoglobin are directly measured by absorption or reflection using 2 to 4 wavelengths of light in the near-infrared region of the hemoglobin spectrum.